Polyalkylene terephthalate resins, such as polyethylene terephthalate or polybutylene terephthalate, have conventionally been used in applications requiring heat resistance. Moreover, polyalkylene terephthalate exhibits excellent mechanical and electrical properties, in addition to favorable processing characteristics. As such, polyalkylene terephthalate resins have been used in a number of diverse fields. Recently, however, the service and end-use environments for polyalkylene terephthalate resins have become more severe thereby creating a need for polyalkylene terephthalate resins of improved properties and/or performance characteristics.
Although polyalkylene terephthalate resins are considered to be "crystalline" resins, they nonetheless include a minor amount of non-crystalline regions. During use, particularly when the polyalkylene terephthalate is subjected to cyclical thermal history conditions (i.e. repeated cycles of high and low temperatures), the non-crystalline regions will crystallize. This crystallization will thereby change the resin's density and will, moreover, give rise to internal stress causing chain cleavage. Thus, the resin's mechanical properties, such as flexibility and impact resistance, are deleteriously reduced. For these reasons, the use of such resins in environments where the flexibility of the resin is important and where the resin is subject to repeated high/low temperature conditions, has been limited due to the adverse affect upon the resin's mechanical properties (i.e. flexibility) that such an environment could cause.
Attempts have been made to lower the crystallinity of polyester resin by the addition of an elastomer so as to address the above problem. However, polyester resin compositions containing an elastomer are disadvantageous in that long-term thermal history conditions cannot be tolerated -- presumably due to the crystalline matrix remaining in the resin. In addition, lower mechanical characteristics (such as abrasion resistance) are exhibited due to the reduced crystallinity of the resin, even though the addition of the elastomer gives some improvement.
What has been needed therefore is a polyalkylene terephthalate resin (e.g. a polyester) that does not lose its flexibility even when subjected to thermal history conditions and thus retains its excellent mechanical and electrical characteristics. It is towards attaining such a resin that the present invention is affected.
Broadly, the present invention is directed to resin compositions comprising an aromatic polyester copolymer having a branched or crosslinked structure and a specific compound (to be described below) that satisfies the above-noted requirements. More specifically, the present invention relates to polyester resin compositions which retain their flexibility even when subjected to cyclic thermal conditions, and comprise, (A) a polyester copolymer prepared by the polycondensation of (a) an aromatic dicarboxylic acid or an ester-forming derivative thereof with (b) an aliphatic glycol and (c) a branch-forming compound wherein said branch-forming compound (c) is contained in an amount of 0.001 to 1.0 mole % based on the component (a), and (B) 0.1 to 10% by weight (based on the total weight of the composition) of a bisoxazoline compound.
Until the present invention, it has been very difficult to provide a polyester resin composition that simultaneously exhibits a number of desirable characteristics, for example, abrasion resistance and flexibility (flexing properties and elongation), together with an ability to retain its initial high elongation and flexing properties for prolonged time periods in heated atmospheres. Accordingly, it is unexpected that compositions comprising a polyalkylene terephthalate containing a branched or crosslinked structure, and a bisoxazoline compound (within a specified ratio) according to the present invention do not lose their flexibility even when subjected to thermal history conditions and, in addition, exhibit significantly improved long-term stability in heated atmospheres.